1. Field of the Invention
This invention relates generally to the field of lithium ion batteries. More specifically, the inventions relates to a process for making high surface area lithium metal phosphates for use as battery cathodes.
2. Background of the Invention
Lithium metal phosphates have emerged as the cathode material of choice for the next generation of lithium-ion secondary batteries used in portable electronics, power tools, and hybrid and electric vehicles due to the fact that lithium metal phosphates have been shown to possess high specific capacities, are more thermally stable than current metal oxide based cathodes, and are environmentally friendly.
LiFePO4 has been characterized as a cathode material. LiFePO4 crystallizes in the olivine structure in which FeO6 octahedra are linked together with PO4 tetrahedra. Lithium is found in the channels created by the linked polyhedra. The advantages of LiFePO4 include its capacity, stability over repeated cycling, and its environmental friendliness. The disadvantage of LiFePO4, as with most phosphate cathode materials, is that the electronic conductivity of LiFePO4 is low. To improve the electronic conductivity necessary for effective batteries, carbon black has been mixed with LiFePO4 to form the battery cathode. LiFePO4 is currently prepared by carbothermal reduction (CTR) or solid state processing. In carbothermal reduction, elemental carbon is added to a mixture of lithium carbonate, iron oxide, and diammonium hydrogen phosphate. As the mixture is heated the carbon reduces the iron and CO or CO2 is produced. The final product is coated with any excess carbon.
Lithium vanadium phosphates have also been characterized as potential battery cathode materials. Li3V2(PO4)3 is found either in the rhombohedral NASICON structure or a monoclinic form. The monoclinic form of Li3V2(PO4)3 has a high specific capacity of 197 mAh/g. The structure contains three crystallographically distinct lithium atoms that can be removed between two and five volts. The first lithium is removed between 3.6 and 3.7 volts (vs. Li/Li+) and the second lithium is removed at 4.08 V (vs. Li/Li+). Removal of these two lithium atoms corresponds to the V3+/V4+ redox couple. The third lithium in Li3V2(PO4)3 is removed at 4.55 V vs. Li/Li+ and corresponds to the V4+/V5+ redox couple.
There are several other vanadium phosphate materials that have potential as lithium-ion battery cathode materials. LiVOPO4 is found in both an α and β-phase. α-LiVOPO4 has been reported to be the thermodynamically stable phase. β-LiVOPO4 converts to α-LiVOPO4 when heated above 750° C. Structurally, both the α- and β-phases contain infinite chains or columns of trans corner-sharing VO6 octahedra. These chains are roughly arranged in a close packing configuration. In the α-structure, the VO6 octahedra are staggered, and the phosphorous and lithium fill the interstitial site between the chains in an alternating pattern. The phosphorous is tetrahedrally coordinated, and the lithium is five-coordinated. In the β-phase, the VO6 octahedra are eclipsed. The phosphorous forms PO4 tetrahedra in alternating sites by linking the corners of adjacent VO6 octahedra. The lithium ions are situated in six coordinate interstitial sites between the PO4 and VO6 polyhedra. Unfortunately, materials such as LiVOPO4 and LiVP2O7 have not been thoroughly investigated due to the fact that a simple synthetic procedure has not been available to prepare high surface area powders with these compounds.
The traditional methods for preparation of these types of compounds include carbothermal reduction, traditional solid-state processing, and glycine nitrate combustion synthesis. In carbothermal reduction, reactants are dry mixed with elemental carbon, pelletized, calcined in an inert atmosphere, ground up, re-pelletized, re-calcined, and ground up a second time. The carbon acts as a reducing agent for reducing the metal(s) in the reactant mixture.
Solid-state processing involves mixing oxide precursors, typically by ball milling, followed by calcining the powder one or more times. However, such solid state processing methods have been unable to produce high surface area powders.
In the glycine nitrate method of combustion synthesis, metal nitrate precursors are dissolved in water with glycine (NH2CH2COOH). The glycine acts both as a chelating agent that provides uniform mixing of the metal precursors, and as the fuel during combustion. The mixture is heated, and the water evaporates until spontaneous ignition occurs. The glycine and nitrates react exothermically producing flames, carbon oxides and nitrogen oxides, and a solid crystalline product. In a typical reaction, the flame temperature during combustion can reach 1800° C. However, glycine nitrate combustion synthesis does not produce the high surface area needed for adequate lithium intercalation.
Consequently, there is a need for a simple and cost-effective process of preparing high surface area lithium metal phosphates using a variety of metals including vanadium.